The Global Positioning System that I use is the Garmin eTrek:

I have set it up so that the GPS co-ordinates that it gives are the in the same format as the many other websites covering Irish sites.  these settings are as follows:

Position format - Irish grid.
Map Datum - Ireland 1965.
Units - Metric.
North Ref - Magnetic.

The GPS is never 100% accurate so I note the accuracy of each reading I take so you know what to factor in if you are having trouble finding a site.  However, this will only happen with barely visible sites like ruined barrows.  I need to thank Megalithomania here for providing this information.